The invention relates to the optical transmission of information and more particularly, to a method and apparatus for improving transmission capabilities over optical fiber transmission systems.
Very long optical fiber transmission paths, such as those employed in undersea or transcontinental terrestrial lightwave transmission systems, which employ optical amplifier repeaters, are subject to decreased performance due to a host of impairments that accumulate along the length of the optical fiber comprising the transmission path. The source of these impairments within a single data channel include amplified spontaneous emission (ASE) noise generated in the Erbium-Doped Fiber-Amplifiers (EDFAs), nonlinear effects caused by dependence of the single-mode fiber""s index on the intensity of the light propagating through it, and chromatic dispersion which causes different optical frequencies to travel at different group velocities. In addition, for wavelength division multiplexed (WDM) systems, where several optical channels might be on the same fiber, crosstalk between channels caused by the fibers nonlinear index must be considered. Typically, it is advantageous to operate long-haul transmission systems at high data rates per channel. For example, multiples of the Synchronous Digital Hierarchy (SDH) standard of 2.5 Gb/s are generally considered useful. Generally speaking, the impairments that limit the system""s performance cause two types of degradations in the received eye pattern, which are related to randomness (caused by noise) and deterministic degradations (or distortions in the received bit pattern). Distortions of the second type are sometimes referred to as Inter-Symbol Interference (ISI). As the bit rates rise into the gigabit per second range it becomes critical to manage those impairments that effect the shape of the received pulses, and to limit the ISI.
Distortions of the received waveform are influenced by the shape of the transmitted pulses and the details of the design of the transmission line. Two signaling formats considered useful in long-haul transmission systems are the non-return-to-zero (NRZ) and solitons formats. The transmission format used in most long-haul lightwave system is the NRZ format because it is easy to generate, detect and process. The name NRZ is applied to this format because it describes the waveform""s constant value characteristic when consecutive binary ones are sent. Alternatively, a string of binary data with optical pulses that do not occupy the entire bit period are described generically as Return-to-Zero or RZ. The two most common examples of RZ signaling pulses are a rectangular pulse that occupies one half of the bit period, and a hyperbolic secant pulse (or soliton) with a pulse width of about ⅕ of the time slot.
Known methods of reducing noise and distortion in lightwave transmission systems include the application of synchronous polarization and phase modulation to the NRZ signaling format (see U.S. Pat. No. 5,526,162), dispersion management of the transmission line, or the use of optical solitons. Scrambling the state-of-polarization of the optical carrier at the bit-rate of the transmitted NRZ signal can greatly improve the transmission performance of long-haul optical amplified transmission systems. In addition to synchronous polarization scrambling, superimposed phase modulation (PM) can dramatically increase the eye opening of the received data pattern. This increase results from the conversion of PM into bit-synchronous amplitude modulation (AM) through chromatic dispersion and nonlinear effects in the fiber. These synchronous polarization/phase modulations techniques were used in a WDM transmission system having a total transmission capacity of 100 Gb/s (20 WDM channels at 5 Gb/s) over 6300 km, as discussed in Bergano, et al., xe2x80x9c100Gb/s WDM Transmission of Twenty 5 Gb/s NRZ Data Channels Over Transoceanic Distances Using a Gain Flattened Amplifier Chain,xe2x80x9d European Conference on Optical Communication (ECOC""95), Paper Th.A.3.1, Brussels, Belgium, Sep. 17-21, 1995.
While these methods have been effective, it is desirable to further reduce distortion to improve the performance of long distance optical transmission systems.
In accordance with the present invention, a method and apparatus is provided that yields improved performance of both single channel and WDM long-distance optical transmission systems by synchronously modulating of the transmitted signal""s amplitude. An amplitude modulator receives an optical signal onto which data has been modulated at a predetermined frequency. The modulator re-modulates the amplitude of the optical signal in a continues fashion with a waveform that is periodic, whose fundamental frequency is equal to the same predetermined frequency at which the data is modulated onto the optical signal. The resulting signal (which is neither a pure NRZ or RZ signal) is more tolerant to the distortions usually found in lightwave transmission systems, thus giving superior transmission performance.
In accordance with one aspect of the invention, an optical transmission system is provided that includes a transmitter, an optical transmission path coupled to the transmitter, and a receiver coupled to the optical transmission path. The transmitter includes an optical signal source for generating an optical signal onto which data is modulated at a predetermined frequency. An amplitude modulator is coupled to the optical signal source for modulating the intensity of the data modulated signal. A clock, which is coupled to the amplitude modulator, has a frequency that determines the frequency of the amplitude modulator. The frequency of the clock is phase locked and equal to the predetermined frequency at which data is modulated.